1. Field of the Invention
The present invention relates to a voltage-controlled oscillation circuit (hereinafter referred to as VCO). More specifically, the present invention relates to a VCO, which is supplied with a control voltage for controlling its oscillation frequency through an impedance element for C-N characteristic (carrier-to-noise ratio) compensation.
2. Description of the Background Art
FIG. 1 is a circuit diagram showing an example of a conventional VCO 1. Referring to FIG. 1, the VCO 1 includes an oscillation stage 4, a buffer stage 5 and an output matching stage 6. The oscillation frequency of the oscillation stage 4 is changed in response to a control voltage Vc which is applied to a control terminal C. The buffer stage 5 prevents the oscillation frequency of the oscillation stage 4 from varying with load fluctuation. The output matching stage 6 attains matching with a next-stage circuit which is connected with an output terminal P, and suppresses higher harmonics.
The oscillation stage 4 includes a resonance circuit 7. This resonance circuit 7 includes a frequency variable varactor diode VD, a coupling capacitor C11 and a resonance inductor L2, and the control terminal C supplies the control voltage Vc to a cathode of the frequency variable varactor diode VD and an end of the coupling capacitor C11 through an impedance element Zvc for C-N characteristic compensation having a line impedance. An anode of the frequency variable varactor diode VD is grounded, while another end of the coupling capacitor C11 is connected to an end of the resonance inductor L2 and an end of the coupling capacitor C10. Another end of the resonance inductor L2 is grounded.
The oscillation stage 4 includes an oscillation transistor Q2, and another end of the coupling capacitor C10 is connected to its base. Further, the base of the oscillation transistor Q2 is supplied with a voltage, which is obtained by dividing a power supply voltage V.sub.B by bias resistors R4 and R5 serially connected between a power supply terminal B and the ground, as a bias voltage. A capacitor C9 is connected between a base and an emitter of the oscillation transistor Q2, while a resistor R6 and a capacitor C8 are connected in parallel between the emitter of the oscillation transistor Q2 and the ground.
The capacitors C9 and C8 form a Colpitts capacitance, and the oscillation transistor Q2 forms a Colpitts oscillator with the capacitors C9 and C8 and the resonance inductor L2, to oscillate at the resonance frequency of the resonance circuit 7.
An oscillation output of the oscillation stage 4 is supplied to the buffer stage 5 through the coupling capacitor C7. The buffer stage 5 includes a buffer transistor Q1, which is supplied with the oscillation output of the oscillation stage 4 in its base. The base of the buffer transistor Q1 is also supplied with a voltage, which is obtained by dividing the power supply voltage V.sub.B by bias resistors R1 and R2 connected in series between the power supply terminal B and the ground, as a bias voltage. A collector of the buffer transistor Q1 is connected to the power supply terminal B through a choke coil L1 which is included in the output matching stage 6.
The output matching stage 6 includes the choke coil L1, a coupling capacitor C1 and an output matching capacitor C2. An end of the coupling capacitor C1 is connected to the collector of the buffer transistor Q1, while another end thereof is connected to the output terminal. On the other hand, an end of the output matching capacitor C2 is connected to the output terminal P, while another end thereof is grounded. A high frequency bypass capacitor C5 is connected between the power supply terminal B and the ground, while another high frequency bypass capacitor C3 is connected between the control terminal C and the ground.
In the VCO 1 having the structure shown in FIG. 1, the capacitance of the frequency variable varactor diode VD included in the resonance circuit 7 is changed in response to the control voltage Vc which is inputted through the impedance element Zvc for C-N characteristic compensation. The resonance circuit 7 resonates on the basis of the coupling capacitor C11, the capacitance of the frequency variable varactor diode VD, and the resonance inductor L2, while the oscillation transistor Q2 oscillates at the resonance frequency thereof. The oscillation output is supplied to the base of the buffer transistor Q1 through the coupling capacitor C7, so that the collector of the buffer transistor Q1 outputs the oscillation output, which in turn is outputted from the output terminal P through the coupling capacitor C1.
In such a VCO, the impedance element Zvc for C-N characteristic compensation is generally connected between the control terminal C and the resonance circuit 7. This impedance element Zvc is formed by an inductive or resistive element to protect the quality factor of the resonance circuit 7 from damping caused by the control voltage Vc, thereby maintaining excellent C-N characteristics. It is possible to attain excellent C-N characteristics when this impedance element Zvc has a high line impedance.
In a miniature VCO which is employed in a cordless telephone, a portable telephone or a pager, the aforementioned impedance element Zvc is mainly formed by a coil, a transmission line such as a stripline, or a resistive element. When a resistive element is employed as the impedance element Zvc, however, the C-N characteristics of the VCO are deteriorated due to the influence of thermal noise. Therefore, the impedance element Zvc is preferably formed by an inductive element.
In order to attain a high impedance when the impedance element Zvc is formed by an inductance element, however, a coil having a large feature size or a stripline which is formed in a wide area is required and hence the VCO cannot be miniaturized.